This invention relates to a process for continuously producing oxymethylene polymers. More specifically, the invention relates to a process for continuously producing an oxymethylene homopolymer or copolymer by polymerizing trioxane or a monomeric mixture containing at least 50 mole% of trioxane in bulk in the presence of a catalyst.
Various processes for the production of oxymethylene polymers have been known to date. For example, a solution polymerization method which is usually carried out batchwise and comprises blowing a purified formaldehyde gas into an inert organic solvent containing a catalyst is well known as a method for producing an oxymethylene homopolymer. This method, however, has the defect of requiring the troublesome purification of the starting formaldehyde.
On the other hand, a polymerization technique starting from trioxane which is solid and easy to handle has advanced, and various processes have been suggested. One of them is a batchwise polymerization method which comprises adding a catalyst to liquefied trioxane and polymerizing it in bulk. According to this method, the reaction product generally agglomerates with the progress of the quick and vigorous polymerization reaction. Hence, it is not easy to pulverize the final polymer, and the product is difficult to take out. In an attempt to avoid these troubles, a liquid-phase batchwise polymerization method was suggested in which a large amount of an inert solvent is used and the polymerization is performed while maintaining the reaction product in the suspended state. This method requires the recovery of the inert solvent used in large amounts, and the use of much solvent leads to products of low molecular weights, or causes a decrease in the rate of polymerization. For this reason, the liquid-phase batchwise polymerization method is not commercially advantageous. Accordingly, a method of polymerizing trioxane in bulk is usually desired to produce an oxymethylene homopolymer or copolymer.
Various methods and apparatuses for polymerizing trioxane in bulk and obtaining oxymethylene polymers as pulverized products have heretofore been suggested.
For example, Japanese patent publication No. 5234/69 discloses a technique of copolymerizing trioxane and ethylene oxide by using a continuous mixing device which is marketed under the tradename "KO-Kneader" and disclosed in the specification of U.S. Pat. No. 2,505,125. This method involves using a cylindrical barrel which includes a screw provided coaxially with the barrel and having a number of ridges that interrupt the screw thread, and reacting the polymerization mixture which moving the polymerized mixture located at the interrupted portions by teeth which project from the inside surface of the barrel. With this method, however, mixing is insufficient, and the reaction mixture does not well contact the inside surface of the cylinder cooled by an external jacket for temperature adjustment. Consequently, the polymerization temperature rises to cause the volatilization of the monomer and/or comonomer and a satisfactory polyoxymethylene copolymer cannot be obtained.
U.S. Pat. No. 2,442,866 discloses a technique involving the use of a "screw-extruder" consisting of a long cylinder with a pair of meshing parallel screws disposed therein as a polymerization apparatus which can increase the mixing efficiency of the reaction mixture. When molten trioxane is polymerized in the screw extruder, the reaction mixture rapidly agglomerates with the progress of the polymerization. This often causes so high a load that the rotation of the screws fails, and the operation becomes virtually impossible.
As one expedient for removing the defects of the screw extruder, Japanese Laid-Open patent publication No. 84890/76 corresponding to U.S. patent application Ser. No. 514,146 filed Oct. 11, 1974 suggests a technique involving the use of a "self-cleaning" type continuous mixing device having the function of automatically scraping the adhering agglomerated reaction mixture as a polymerization reactor. The polymerization reactor used in this method consists of a long case including a pair of parallel shafts each of which is equipped with many elliptical plates, the elliptical plates being provided such that when the shafts are rotated, the ends of the long axes of the elliptical plates attached to one shaft always rub the surfaces of the elliptical plates attached to the other shaft. The use of such a self-cleaning polymerization reactor is favorable in actual practice because the final product discharged from it has a relatively small particle size, but has the defect that the conversion of monomer is low. Attempts to increase the conversion of monomer in this method have not produced satisfactory results. For example, if the concentration of catalyst is increased, the molecular weight of the final polymer decreases. If the reaction temperature is raised, unusual reactions including depolymerization take place, and the heat generated by these reactions cause the instantaneous volatilization of monomers. After all, satisfactory polymer products cannot be obtained. If the residence time in the mixing device is prolonged, the output of the polymer decreases markedly. In order to increase the hold-up of the mixing device and prolong the residence time, it is necessary to increase L/D (the length-to-diameter ratio). Increasing L/D, however, results in a tremendous increase in the cost of building the device, and also causes operational and maintenance troubles such as the poor contacting of elliptical plates caused by the flexure of the shafts, and the damage of the inside surface of the device caused by the elliptical plates. Thus, a mixing device having a high L/D is not practical.
As is clear from the foregoing description of the prior art, the production of an oxymethylene homopolymer or copolymer from trioxane as a main starting material by bulk polymerization has two important problems to solve, one to prevent the agglomeration of the reaction mixture and obtain the pulverized product, and the other to increase the mixing efficiency of the reaction mixture and raise the conversion of monomer. The prior art techniques, however, have failed to solve the two problems at the same time.
The present inventors made extensive investigations about the continuous bulk polymerization of a starting monomer consisting mainly of trioxane in order to solve the above problems. These investigations led to the conclusion that with the phase change from liquid monomer to solid polymer, the polymer tends to adhere strongly to the reactor wall and the stirrer, and in order to prevent the agglomeration of the reaction mixture and obtain a pulverized polymer, the use of a self-cleaning type polymerization reactor is essential. However, to achieve a high monomer conversion of, say, 95 to 100% intended by the present invention, the residence time of the reaction mixture in the reactor should be adjusted to at least about 40 to 60 minutes. To give such a long residence time, the L/D of the reactor needs to be made very high. Consequently, the method cannot avoid the inherent disadvantages caused by high L/D as described hereinabove. In order to remove this inconsistency, the present inventors furthered their investigations, and found that (1) in the bulk polymerization of a starting monomer comprising trioxane as a main ingredient, the state of the reaction mixture changes depending upon the conversion of monomer, and as a typical example, the reaction mixture is liquid if the conversion is up to about 20%, and it is a slurry at a conversion of about 20 to 30%, in the form of unbaked bread at a conversion of about 30 to 40%, a wet coarse powder at a conversion of 40 to 50%, a non-tacky powder at a conversion of about 50 to 60%, and a hard powder at a conversion of more than about 60%; and (2) the tackiness of the reaction mixture is most outstanding when it is in the form of slurry, and it shows considerable tackiness when it is in the form of unbaked bread, but after the reaction mixture has become a wet coarse powder, it does not show tackiness which is substantially detrimental. From this new observation, the present inventors concluded that to avoid the adhesion of the reaction mixture to the reactor wall and the stirrer, it is not altogether necessary to use an expensive self-cleaning type reactor over the entire period of the polymerization reaction, but the adhesion of the reaction mixture can be effectively prevented by using a self-cleaning type reactor only in the early stage of the reaction, and using a cheaper nonself-cleaning type ordinary reactor equipped with a stirrer in the later stage of the reaction.
The method of performing polymerization using such two types of reactors has increased utility because of the fact described below. In the bulk polymerization of a starting monomer consisting mainly of trioxane, the rate of reaction is high in the early stage of the reaction where the conversion is low, but abruptly decreases with increasing conversion. It has been found by an analysis made by the present inventors of the rate of reaction in a polymerization reaction under certain reaction conditions, a conversion of 70% is achieved after a lapse of 10 minutes, and in order to increase the conversion to almost 100%, a time of at least 80 minutes is required. Under these circumstances, if it is sufficient to advance the polymerization reaction to a conversion which will remove the trouble of the adhesion (in the above-given example, a conversion of about 40%), the residence time of the reaction mixture in the reactor is very short. Hence, even if such a reaction is carried out in a self-cleaning type reactor, the L/D of the reactor is very low, and therefore, the process is fully feasible. From the standpoint of commercially acceptable L/D, it is permissible to carry out the polymerization reaction in a self-cleaning type reactor until the conversion reaches about 70%. However, with increasing conversion, the residence time in the reactor increases abruptly, and the required L/D becomes exceedingly high. It is disadvantageous therefore to perform the polymerization in a self-cleaning type reactor to a conversion of more than 70%. In practice, it is difficult to increase the conversion to at least 90% using the self-cleaning type reactor.
As will be seen from the foregoing statement, in the bulk polymerization of a starting monomer consisting mainly of trioxane, the prevention of the adhesion of the reaction mixture is important in the early stage of the reaction, and the pulverization of a hard powder is important in the later stage of the reaction. Since however, the self-cleaning type reactor has a good function of scraping the adhering polymer, it is very suitable for use in the early stage reaction. On the other hand, because an ordinary reactor including stirring vanes generally permits a very large hold-up as compared with self-cleaning type reactors, it is very suitable for use in the later-stage reaction which requires a longer residence time.
The present invention was accomplished on the new findings of the present inventors described hereinabove. It is an object of this invention to convert a starting monomer containing at least 50 mole% of trioxane continuously to an oxymethylene homopolymer or copolymer by bulk polymerization. Another object of the invention is to substantially complete the conversion of the starting monomer to the polymer. A still another object of the invention is to continuously withdraw an oxymethylene homopolymer or copolymer as a powder after substantial completion of conversion of the starting monomers to the polymer.